In disc brakes used to brake a motor vehicle, a pair of pads are disposed so as to sandwich a rotor which rotates together with a wheel, and when the brakes are applied, both the pads are pressed against both axial side surfaces of the rotor. There are two types of basic constructions for such disc brakes: one is a construction with a floating caliper, and the other is a construction with an opposed-piston fixed caliper. In the case of either of the constructions, the rotor which rotates together with the wheel is strongly sandwiched from both axial sides thereof by the pair of pads when the brakes are applied. In both the pads, a lining is additionally attached to a front surface of a pressure plate which has sufficient rigidity. Then, when the brakes are applied, a back surface of the pressure plate is pressed, whereby a front surface of the lining is brought into frictional contact with both axial side surfaces of the rotor. When referred to in this description and the claims, axial direction, circumferential direction and radial direction mean an axial direction, circumferential direction and radial direction of a rotor, respectively, in such a state that a disc brake pad assembly is assembled to a disc brake unless otherwise described. In addition, a circumferential edge portion means an inward circumferential edge portion or an outward circumferential edge portion with respect to the radial direction of the rotor.
When the brakes are applied, an abutment portion where the axial side surfaces of the rotor and the front surfaces of the linings of the pads are brought into abutment with each other and which constitutes a portion where a frictional force is exerted is offset axially from an abutment portion where the pressure plates and supports or a caliper are brought into abutment with each other and which constitutes an anchor portion which bears brake torque which is exerted on both the pads by at least an amount equaling a total thickness of the linings of both the pads (the frictional portion is offset from the brake torque bearing portion). Then, a moment is exerted on both the pads in a direction in which a leading side of the rotor approaches (falls towards) both the pads based on the offset equaling the thickness of the linings, and this easily makes the orientations of both the pads unstable. In case the orientations of both the pads become unstable when the brakes are applied, it becomes difficult that both the pads behave smoothly, and these pads are caused to vibrate, whereby noise referred to as a “brake squeal” is easily generated or the degree of partial wear of the linings easily becomes remarkable.
In order to mitigate the brake noise or the partial wear described above, it is conventional and widely accepted practice to sandwich a shim plate between a back surface of a pressure plate which makes up a pad and a distal end face of a piston which constitutes a pressing surface which presses the back surface or an inner surface of a caliper claw portion. In a shim plate like this, although a single-plate configuration is adopted in which only a single shim plate is used, in order to improve the effect of suppressing the brake squeal or the partial wear, a double-plate configuration is also widely adopted in which an inner shim plate and an outer shim plate are fittingly superposed on each other. Additionally, a construction has also been known conventionally in which an inner and outer shim plates are assembled together so as to be displaced relative to each other in a circumferential direction while being prevented from being separated from each other in a thickness direction.
Patent Literature 1 describes a construction as shown in FIGS. 13 to 15 as a disc brake pad assembly which has the function described above. In the disc brake pad assembly of the conventional construction, a combined shim plate 5 which is made up of an inner shim plate 3 and an outer shim plate 4 is mounted on a back surface of a pressure plate 2 which makes up a pad 1. In the pad 1, a lining 6 is additionally attached and fixed to a front surface of the pressure plate 2 (which is a surface which faces a side surface of a rotor (not shown) when assembled to a disc brake) with a large fastening force so as not to be forced to deviate by brake torque exerted when the brakes are applied. The inner shim plate 3 is made of a metallic plate which is a stainless steel plate, a rubber-coated stainless steel plate or the like and includes a flat plate-shaped inner main body portion 7 and a plurality of inner locking pieces 8a, 8b, 8c. A front half portion of each of the inner locking pieces 8a, 8b, 8c has substantially an “L” shape in which an axial middle portion is bent towards a circumferential edge portion of the pressure plate 2a. 
Additionally, a plurality of through holes 9, 9 are formed in the inner main body portion 7 so as to hold a grease therein. In addition, of radially inward and outward circumferential edge portions of the pressure plate 2, a locking recess portion 10 is formed in a circumferential central portion of the radially outward edge portion, and a pair of step portions 11, 11 are formed at portions of the radially inward edge portion which lie close to circumferential ends thereof. In the inner locking pieces 8a, 8b, 8c of the inner shim plate 3, the radially outward inner locking piece 8a is brought into engagement with the locking recess portion 10, while the radially inward inner locking pieces 8b, 8c are brought into engagement with the step portions 11, 11, whereby the pressure plate 2 is held from both radial sides thereof by the inner locking pieces 8a, 8b, 8c. In this state, the inner shim plate 3 is attached to a back surface side of the pressure plate 2 in such a state that circumferential and radial displacements thereof are restricted (actually prevented).
In addition, the outer shim plate 4 is made of a metallic plate which is a stainless steel plate or the like and includes a flat plate-shaped outer main body portion 12 and a plurality of outer locking pieces 13a, 13b, 13c. A front half portion of each of the outer locking pieces 13a, 13b, 13c also has substantially an “L” shape in which an axial middle portion is bent towards a circumferential edge portion of the pressure plate 2a. In the outer shim plate 4 described above, the outer main body portion 12 is fittingly superposed on the inner main body portion 7 with the outer locking pieces 13a, 13b, 13c fittingly superposed on the inner locking pieces 8a, 8b, 8b, respectively. As this occurs, projecting portions of inner surfaces (surfaces which face the circumferential edge portions of the pressure plate 2) of the front half portions of the outer locking pieces 13a, 13b, 13c are brought into elastic engagement with recess portions on outer surfaces (surfaces opposite to the circumferential edge portions of the pressure plate 2) of the front half portions of the inner locking pieces 8a, 8b, 8c, respectively. In this state, the outer shim plate 4 is assembled to the inner shim plate 3 so as to be displaced in a circumferential direction while being restrained from being separated from the inner shim plate 3 in the thickness direction. Because of this, a circumferential widthwise dimension of the outer locking piece 13a is made smaller than circumferential widthwise dimensions of the locking recess portion 10 and the inner locking piece 8a, and a space between circumferential outer edges or opposite side edges of the outer locking pieces 13b, 13c is made smaller than a space between the step portions 11, 11.
In the case of the conventional construction having the configuration described above, since the inner locking pieces 8a, 8b, 8c are fittingly superposed on the outer locking pieces 13a, 13b, 13c, respectively, the projecting amount of the outer locking pieces 13a, 13b, 13c from the circumferential edges of the pressure plate 2 is increased. As a result of this, the prevention of interference of the outer locking pieces 13a, 13b, 13c with other constituent members of the disc brake, for example, a caliper needs to taken into consideration. Although the installation space of the disc brake is limited, the constituent members including the caliper and the like are required to have a large magnitude of rigidity. Because of this, the fact that the prevention of the interference needs to be taken into consideration becomes disadvantageous from the viewpoint of ensuring the degree of freedom in designing the disc brake.